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Abstract

Persistent weak layers present a particular challenge for avalanche forecasters due to their
long lifetime and the difficulty of obtaining observations once they are deeply buried. Melt-freeze crusts are one type of persistent weak layer that is often associated with deep slab avalanches during late winter or spring. This study seeks to improve the understanding of
the thermal and structural properties of melt-freeze crusts by tracking them from formation
through to isothermal conditions in the spring.
Specific Surface Area (SSA) was tracked weekly using near-infrared digital photography
for nine natural crusts and four cold lab crusts during the winters of 2008-09 and 2009-10.
Image analysis techniques were adapted from existing methods in order to track the mean
SSA for specific structures within crusts, as well as vertical profiles of SSA across crust
boundaries. Few temporal trends were identified even in the presence of strong diurnal slope
normal temperature gradients, but the ratio of mean SSA between crusts and adjacent layers
did reveal relative changes in the structure.
The thermal conductivity was tracked for six natural and five cold lab crusts during
the winter of 2009-10 using a heated needle probe. Thermal conductivity of two cold lab
crusts increased during freezing and subsequently decreased in the presence of strong vertical
temperature gradients, while that of natural crusts had no discernible trends under weak
temperature gradients. Trends of increasing thermal conductivity in adjacent layers were
well correlated with increasing density as in previous studies but with a positive offset that
may be attributable to the warmer snow temperatures in this study relative to past studies.
The SNOWPACK model was used to model the formation and evolution of spatially
uniform crusts at a flat study plot as well as on a virtual slope. Persistent model cold
temperature biases were found on the virtual slope, which resulted in delays in settling and
densification relative to observations. A warm model bias was found for the flat simulation,and settling and layer water content exceeded what was observed. Both biases were likely related to meteorological inputs.